Electrophotography instantaneously provides high-quality images, and is therefore widely used for copiers, printers, printing machines. As the electrophotographic photoreceptor that is the core in electrophotography (hereinafter this may be referred to as “photoreceptor” when appropriate), widely used is one that uses an organic photoconductive substance having the advantages of being free of pollution, easy to form into films and easy to produce.
In planning the photoreceptor, one important characteristic thereof is lightfastness. In general, the photoreceptor is used inside a copier or a printer as shielded from light therein. However, in machine assembling, or in clearing a paper jam in a jammed machine, or for machine maintenance in exchanging a photoreceptor unit at the end of its life for a fresh one, the photoreceptor is inevitably exposed to external light (fluorescent lamp or sunlight).
The intensity of the external light is decidedly higher than the exposure intensity for image formation inside the machine, and as containing many short-wavelength rays, the external light would greatly damage the photoreceptor. This is because, inside the photoreceptor exposed to light, a large amount of charge traps form, therefore causing, in many cases, reduction in charge potential or significant increase in residual potential.
Heretofore for preventing photoreceptors from being damaged by external light, and for example, as the lighting in machine assembling, used is an yellow lamp having less influence on the photoreceptors, or in opening the inside of machines, a douser or the like is employed for protecting the photoreceptor as much as possible from being exposed to light.
On the other hand, for preventing the residual potential of the photoreceptor itself from increasing during exposure to light, for example, various additives that are to be incorporated in a charge transport layer have been investigated (for example, see PTL 1 to 4).